In an ideal sound reproducing system acoustical volume current corresponds to electrical signal current. In other words, velocity imparted to the air molecules at the surface of the speaker cone is directly proportional to the pressure that existed at the recording microphone. Engineers refer to such a system as linear i.e. output is a constant multiple of input. An ideal linear system reproduces signal without distortion.
Modern high-fidelity amplifiers are quite linear, producing perhaps one part distortion to a thousand parts of signal. Loudspeakers, on the other hand, fall far short of this performance and have been called the weakest link in the system. It would seem that further improvement of the power amplifier would be unproductive until speaker performance catches up. And this is not likely to happen.
As a practical consequence we find that a particular loudspeaker will sound the same when driven by a common program from a variety of amplifiers, but the reverse is not true. Different speakers driven by the same amplifier and program will sound quite different. While amplifier design has matured, innovation and new entries are common in the speaker industry, each attempting an improvement in linearity.
The causes of speaker deficiencies are complex, but these deficiencies subject the listener to dissonant combinations of tones that affect the listener by what has been referred to as "speaker fatigue". Many users prefer to advance the bass tone control on their equipment--a practice that tends to diminish this effect.
Much of the improvement in amplifier performance came about through the process of sampling its output to provide information with which to alter its input (feedback).
Attempts to do the same for the speaker have been frustrated because its output is not accessible but separated from the speaker terminals by the resistance of the voice-coil. This resistance is in series with the amplifier. It, therefore, is not possible to make a direct connection to the operating part of the speaker; this leaves the speaker free to display its own set of peculiar characteristics to color the sound, often very badly.
Over the past forty years various schemes have been devised to diminish the effects of voice-coil resistance, as evidenced by their patents, but none appear to be a commercial success. They employed electrical bridges, microphones, accelerometers, capacitors, etc. to monitor the output of the loudspeaker.
The bridge method recognizes that the speaker contains its own best velocity measureing device--the voice-coil acting as a generator. But eliminating the effects of the voice-coil resistance by this means is too brutal, causing a new set of disturbances.